Foto: Olga Gordeeva/ Mostphotos.

The sea is not only humanity's main ally in the fight against climate change. It is also an important lower for some harmful pollutants.

In a new Norwegian-Swedish study, researchers have investigated how hydrophobic organic contaminants such as polycyclic aromatic hydrocarbons (PAHs) and polychlorinated biphenyls (PCBs) bind to organic carbon in the bottom sediments of the Baltic Sea.

– The seabed acts as a sink for this type of environmental toxins, which have a very low water solubility. The bottom sediment will be a kind of storage place, and a fairly safe one as well, says Anna Sobek, professor at the Department of Environmental Science at Stockholm University, and one of the researchers behind the study.

Binds better to marine carbon

Sedimentation takes place when the toxic chemicals bind to particles of organic carbon in the sediment. Over time, they then sink further and further down into the bottom sediment and are eventually so far down that they are buried there for good.

However, the ability of carbon particles to capture environmental toxins seem to depend on whether they consist of so-called marine carbon, i e. organic material that originates in the sea (for example dead phytoplankton), or land-based carbon that originates from organic material from land.

The researchers' measurements show that PCBs and PAHs bind up to ten times better to marine carbon compared to land-based carbon. The largest effect was detected for the largest PCBs analyzed in the study.

– We do not know exactly how the carbon cycle will change in the Baltic Sea as a result of climate change. But we know it will change. As a result, the ability of the bottom sediments to retain these environmental contaminants is likely to deteriorate, says Anna Sobek.

Affects the coastal zone

Anna Sobek, Associate Professor at the Department of Environmental Science at Stockholm University.

Climate change will probably bring more precipitation and heavy rainfall, which increases runoff from land via rivers, streams and watercourses. As a result, more land-based coal ends up in the sea, which according to this study impairs the sediment's ability to bind and retain the environmental toxins.

– The effect will be greatest in the coastal zone because this is where most land-based coal from the runoff ends up, says Anna Sobek.

Molecular structure is probably crucial

The researchers performed measurements at two different locations in a bay in the Finnish archipelago. The first location was near a river that carries a lot of organic material from land. The second location was further out towards the open sea, and was therefore dominated by marine carbon.

In the part of the bay that had the most marine carbon, the bottom sediment was between two and ten times better at binding PCBs and PAHs compared to the location closer to the estuary.

– We do not know exactly why the contaminats bind better to marine carbon. This is probably related to the molecular structure and polarity of the carbon particles. This is definitely something that research should take a closer look at, says Anna Sobek.

"One of few ways to get rid of PCBs"

PCBs and PAHs are extremely long-lived and still belong to the most common environmental toxins in the Baltic Sea. They are carcinogenic and can, among other things, damage the immune system, metabolism and reproduction in both humans and animals. When they occur free in the water mass, they can be taken up by, for example, plankton and then travel further in the food web, to fish, birds, mammals and humans.

PAHs are formed during incomplete combustion, and come, for example, from road traffic with fossil-powered vehicles and emissions from heating plants. PCBs were banned in the 1970s almost all over the world but still occur in the environment, both on land and in water. The spread of both PAHs and PCBs to the sea takes place mainly via the air, but to some extent also via runoff from land.

– Sediment, and especially deep-sea sediment, is one of the most important sinks for PCBs. It is one of few ways we have to actually get rid of the PCBs that are still circulating in the environment. If this function becomes less effective, it will matter for how long we will keep these substances in the environment, says Anna Sobek.

– We simply have to get better at reducing emissions of these long-lived chemicals.

Text: Henrik Hamrén

Further reading

Study: Effects of Organic Carbon origin on Hydrophobic Organic Comntaminant Fate in the Baltic Sea